Summary

Optimize java.text.DecimalFormat.format by taking advantage of
numerical properties of integer and floating-point arithmetic to
accelerate cases with two or three digits after the decimal point.

Goals

Make common usages of DecimalFormat faster.

Success Metrics

At least a 2X speedup on micro-benchmarks of interest.

Description

The decimal format conversions of most interest are for those with two
and three digits after the decimal point. Rather than perform expensive
floating-point divisions to isolate and round the fractional digits, a
floating-point multiply of the fractional portion by 100.0 or 1000.0 can
be performed, the product converted to an integer value, and then the
resulting integer converted to decimal. While this process is faster,
care must be taken to avoid double-rounding and other numerical hazards.

Case analysis using properties of which fractional decimal values can be
exactly represented in binary reduces rounding post-multiply to a look up
table style computation.

Testing

Besides running existing JCK and regression tests, new tests targeting
the boundary cases of the optimized code path will be developed. In
addition, the performance of the code will be assessed using
micro-benchmarks on contemporary hardware platforms.

Risks and Assumptions

Exploratory work with the optimized implementation revealing a
long-standing numerical bug in DecimalFormat: some near half-way cases
did not round correctly. The specification of DecimalFormat is being
amended in Java SE 8 to clearly require correct rounding all the time.

Impact

Compatibility: On JDK 7, (correct) but altered numerical behavior
will only be enabled under an aggressive optimization flag to limit
behavioral compatibility impact in that release train. In Java SE 8,
the correct numerical behavior will always be required by the
specification.

Performance/scalability: Many common cases will go faster with the
new algorithm; other cases will fall back to using the older code.